The internal body pressure balances the external atmospheric pressure.
Explanation
Atmospheric pressure is the force exerted per unit area due to the weight of the column of air above a surface. At sea level, this pressure is approximately $10^5$ Pascals ($1 \text{ atm}$). For an average human adult with a surface area of about $2 \, m^2$, the total force exerted by the atmosphere is immense, roughly equivalent to a weight of 20,000 kg.
Detailed Analysis:
- The skeletal structure is rigid enough to completely resist atmospheric pressure. is Incorrect: While the skeletal system provides structural support, it is not designed to withstand such immense crushing forces in isolation. Without internal pressure, soft tissues and organs would collapse under the weight of the atmosphere.
- The internal body pressure balances the external atmospheric pressure. is Correct: The human body is largely composed of fluids (such as blood and water) and cavities containing gases. Fluids are generally incompressible and transmit pressure. The internal pressure within the body’s cells and circulatory system is maintained at a level roughly equal to the external atmospheric pressure. This state of equilibrium ensures that the net force acting on the body surface is zero, preventing collapse.
- Atmospheric pressure acts only on the horizontal surfaces of the body. is Incorrect: Atmospheric pressure is a fluid pressure. According to Pascal’s principle, pressure in a fluid acts equally in all directions, not just on horizontal surfaces.
- The surface area of the human body is too small to experience significant force. is Incorrect: The surface area of a human body is significant ($1.5$ to $2 \, m^2$). Given the magnitude of atmospheric pressure ($101,325 \, N/m^2$), the force experienced is very large, not small.
Key Takeaway:
The human body does not collapse under atmospheric pressure because the internal pressure exerted by body fluids and dissolved gases balances the external atmospheric pressure.